We have examined the changes in the activities of the different types of
alpha-D-mannosidase when fibroblasts from patients deficient in the lysosomal form of the
enzyme are cultured together with normal lymphocytes. Our results show that whereas the
mannosidosis cells acquired high levels of this
enzyme, the activities of both the Golgi and the endoplasmic reticulum forms of
alpha-D-mannosidase remained the same as in the fibroblasts cultured alone in the absence of lymphocytes. The increase in the activity of the lysosomal
enzyme in the cocultured fibroblasts was not affected by the presence of
mannose 6-phosphate or alpha-
methyl mannoside, inhibitors of receptor- and
lectin-mediated uptake of lysosomal
enzymes, respectively, but it did require cell-to-cell contact. Ion-exchange HPLC and electrophoresis in
polyacrylamide gradient
gels showed that the acquired
enzyme had the same elution profile and molecular size as the lysosomal form of the
enzyme present in the lymphocytes. Immunoprecipitation studies using antibody specific for the lymphocyte type of lysosomal
alpha-D-mannosidase confirmed that the increased activity in the cocultured
mannosidosis cells resulted from the acquisition of the lymphocyte
enzyme. Cytochemical examination revealed, however, that the transferred lymphocyte
enzyme was localized in cytoplasmic organelles in the peripheral regions of the recipient fibroblasts. These results show that lymphocytes transfer only the lysosomal form of
alpha-D-mannosidase during cell-to-cell contact with
mannosidosis cells.